Several recent studies have reported a positive relation between income inequality and mortality. The association has been observed in US metropolitan areas and states and, to varying degrees, in international studies.1–3
The relation remains intact when different measures of income inequality are used. The critical question is how this relation should be interpreted.
Three competing interpretations have been advanced. Wilkinson believes that income inequality produces psychosocial stresses for individuals placed at lower ranks of the socioeconomic hierarchy.4–6
Continuous stress due to deprivation of status will lead to deteriorating health and higher mortality over time. The fact that median or per capita household income cannot account for the relation has been taken as evidence that “relative income,” or income inequality, is more important than absolute income for human health and longevity.
Gravelle argues that the correlation between income inequality and mortality may be artefactual in part.7
He shows mathematically that the aggregate relation is consistent with a negative, curvilinear relation between income and the probability of dying for individuals. Wolfson et al's clever test of Gravelle's hypothesis indicates, however, that the individual relation between income and mortality cannot fully account for the aggregate relationship.8
The “neo-material” interpretation asserts that income inequality reflects individual and community forms of absolute deprivation. Lynch et al argue that poorer individuals disproportionately experience health taxing events and lack of resources throughout their lives.9
They live in deprived communities characterised by “underinvestment” in the social and physical infrastructure. Both forms of deprivation produce cumulative wear and tear. The experience depletes health, resulting in higher mortality for those in lower socioeconomic strata. The aggregate effect is that societies with increasing income inequality will experience higher mortality than they would otherwise. Lynch et al suggest that material conditions may be sufficient in explaining the relation between income inequality and mortality.
The neo-material interpretation gives only a broad indication of which material circumstances are important. Kaplan et al's analysis of US states, however, suggests some potential answers.2
They report that income inequality is significantly correlated with certain risk factors (homicide rates and unemployment rates), social resources (food stamps and lack of health insurance), and measures of human capital (educational attainment). The substantial correlations with some measures of human capital imply that income inequality may not have a direct effect on mortality. Instead, income inequality may reflect the effects of other socioeconomic variables that are also related to mortality. Among those variables, the contribution of formal education deserves most attention since it typically precedes work and income. It is also related to mortality.
Higher educational degrees are typical prerequisites for highly compensated work in the United States and other industrialised nations. According to US census data for the year 1998, the median earnings of adult, year round workers with professional degrees are about four times higher than those of adults who had not completed high school.10
Thus, the level of education ought to be correlated with cumulative income, which is the basis for measuring income inequality.
In addition, more schooling seems to extend life.11–14
In econometric studies years of schooling typically had a stronger negative effect on age adjusted mortality than per capita income when other measures were controlled for. Therefore, the association between income inequality and mortality found in aggregate studies may be partially the result of variation in educational attainment. I tested this hypothesis using data for the US states, which have shown substantial associations between measures of income inequality measures and age adjusted mortality.